G.f. A(x) satisfies A(x) = 1 + x/(1-x^3)^2 * A(x)^3.

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =57a(5) =285a(6) =1500a(7) =8195a(8) =46023a(9) =264063a(10) =1541227a(11) =9121923a(12) =54619314a(13) =330265437a(14) =2013832533a(15) =12369068985a(16) =76455159257a(17) =475232409786